On Pentecost God's spirit manifested its presence in the disciples by giving them the power to speak in human languages they did not know (often referred to in Pentecostal circles as speaking in tongues).
When the label Pentecostal is applied to religious-related groups or customs it usually designates that they believe all Christians should partake of the same manifestations of God's spirit as was displayed in Acts 2. Emphasis is usually placed on experiencing the "gift of tongues," which to some in the movement is speaking unknown or spiritual words that require interpretation in order to understand their meaning.
These people also emphasize the teaching of a "full gospel" or "foursquare gospel." The four squares of such a Gospel revolves around four actions Jesus has or will soon take.
The first square is that Jesus saves according to John 3:16. The second is he baptizes with the Holy Spirit according to Acts 2:4. The third is he heals bodily according to James 5:15. The last of the Pentecostal four squares is that Jesus will come again (Second Coming) to receive those who are saved (usually as part of a rapture) according to 1Thessalonians 4:16 - 17.
The term Charismatic is sometimes used to refer to the same people and behaviors as the term Pentecostal. Many students of the Bible and religious history in general, however, define this group of people as those who developed and came out of the Holy Spirit movement that began around 1900 AD.
Roughly speaking, in the year 2000 around 115 million people worldwide were considered Pentecostal. Some of the major denominations include The Assemblies of God, Church of God in Christ, The Church of God (Cleveland, Tennessee), International Church of the Foursquare Gospel, International Pentecostal Holiness Church and many others.